[ResponseOps][Alerting] Elasticsearch query rule type allows SIZE: 0, but flags as error on re-edit

x-pack/plugins/stack_alerts/public/alert_types/es_query/validation.test.ts

@@ -158,6 +158,20 @@ describe('expression params validation', () => {
     );
   });
 
+  test('if size property is 0 should not return error message', () => {
+    const initialParams: EsQueryAlertParams<SearchType.esQuery> = {
+      index: ['test'],
+      esQuery: `{\n  \"query\":{\n    \"match_all\" : {}\n  }\n`,
+      size: 0,
+      timeWindowSize: 1,
+      timeWindowUnit: 's',
+      threshold: [0],
+      timeField: '',
+      excludeHitsFromPreviousRun: true,
+    };
+    expect(validateExpression(initialParams).errors.size.length).toBe(0);
+  });
+
   test('if size property is > 10000 should return proper error message', () => {
     const initialParams: EsQueryAlertParams<SearchType.esQuery> = {
       index: ['test'],

x-pack/plugins/stack_alerts/public/alert_types/es_query/validation.ts

@@ -5,7 +5,7 @@
  * 2.0.
  */
 
-import { defaultsDeep } from 'lodash';
+import { defaultsDeep, isNil } from 'lodash';
 import { i18n } from '@kbn/i18n';
 import { ValidationResult, builtInComparators } from '@kbn/triggers-actions-ui-plugin/public';
 import { EsQueryAlertParams, ExpressionErrors } from './types';
@@ -63,7 +63,7 @@ export const validateExpression = (ruleParams: EsQueryAlertParams): ValidationRe
     );
   }
 
-  if (!size) {
+  if (isNil(size)) {
     errors.size.push(
       i18n.translate('xpack.stackAlerts.esQuery.ui.validation.error.requiredSizeText', {
         defaultMessage: 'Size is required.',

x-pack/plugins/triggers_actions_ui/public/common/expression_items/threshold.test.tsx

@@ -140,4 +140,27 @@ describe('threshold expression', () => {
     wrapper.update();
     expect(wrapper.find('input[data-test-subj="alertThresholdInput"]').length).toEqual(1);
   });
+
+  it('is valid when the threshold value is 0', () => {
+    const onChangeSelectedThreshold = jest.fn();
+    const onChangeSelectedThresholdComparator = jest.fn();
+    const wrapper = shallow(
+      <ThresholdExpression
+        thresholdComparator={'>'}
+        threshold={[0]}
+        errors={{ threshold0: [], threshold1: [] }}
+        onChangeSelectedThreshold={onChangeSelectedThreshold}
+        onChangeSelectedThresholdComparator={onChangeSelectedThresholdComparator}
+      />
+    );
+    expect(wrapper.find('[data-test-subj="alertThresholdInput"]')).toMatchInlineSnapshot(`
+    <EuiFieldNumber
+      data-test-subj="alertThresholdInput"
+      isInvalid={false}
+      min={0}
+      onChange={[Function]}
+      value={0}
+    />
+    `);
+  });
 });

x-pack/plugins/triggers_actions_ui/public/common/expression_items/threshold.tsx

@@ -17,6 +17,7 @@ import {
   EuiFieldNumber,
   EuiText,
 } from '@elastic/eui';
+import { isNil } from 'lodash';
 import { builtInComparators } from '../constants';
 import { Comparator } from '../types';
 import { IErrorObject } from '../../types';
@@ -145,14 +146,14 @@ export const ThresholdExpression = ({
                 ) : null}
                 <EuiFlexItem grow={false}>
                   <EuiFormRow
-                    isInvalid={errors[`threshold${i}`]?.length > 0 || !threshold[i]}
+                    isInvalid={errors[`threshold${i}`]?.length > 0 || isNil(threshold[i])}
                     error={errors[`threshold${i}`]}
                   >
                     <EuiFieldNumber
                       data-test-subj="alertThresholdInput"
                       min={0}
                       value={!threshold || threshold[i] === undefined ? '' : threshold[i]}
-                      isInvalid={errors[`threshold${i}`]?.length > 0 || !threshold[i]}
+                      isInvalid={errors[`threshold${i}`]?.length > 0 || isNil(threshold[i])}
                       onChange={(e) => {
                         const { value } = e.target;
                         const thresholdVal = value !== '' ? parseFloat(value) : undefined;